The present invention relates to a circuit arrangement for connecting a subscriber's set to the subscriber line in an electrical two-way communication network.
When terminating a subscriber line or connecting a subscriber's set to such a line in a system of this type, there exists the requirement for the most accurate and constant matching of the characteristic impedance of this line whereby the phase shift caused by the length of the line must be compensated. The simplest solution to this problem is to terminate the line with a constant ohmic resistance of the value of the characteristic impedance of the line, with an additional blind component to compensate for the phase shift caused by the line. However, this solution is available only if the subscriber's set is not fed through the subscriber line.
In almost all Post Office owned telephone networks, however, the subscriber is fed from the central office. Due to the different line lengths, there result rather different supply currents to various subscriber sets. For example, when a screen text modem is connected to a telephone subscriber's line, with supply currents of between 17 and 22 mA, a terminating d.c. impedance of .ltoreq.400 ohms must be maintained; for supply currents from 22 to 60 mA, a terminating d.c. impedance of .ltoreq.300 ohms must be maintained; and for the transmission of alternating current modem signals, a terminating impedance of 600 ohms. Without the aid of inductive and/or transformer components, these requirements cannot be met.